Composites are attractive to the aerospace industry because of their high strength and rigidity and low weight. Composite structures such as skin and stiffeners may be constructed by laying up plies of reinforcing fibers on a mandrel, forming an air-tight envelope such as a bagging film over the reinforcement, infusing resin into the “bagged” reinforcement, and curing the resin-infused reinforcement.
A first type of resin infusion system includes a liquid resin in a resin inlet tank. Resin infusion is performed by conveying the liquid resin and injecting the liquid resin through the envelope. Once underneath the envelope, the liquid resin is distributed to the reinforcement via distribution media (typically more than one ply of a permeable fabric). The rate of resin infusion may be controlled by varying permeability of the distribution media, partially restricting the flow of resin by pinching or otherwise restricting an inlet line, or controlling atmospheric pressure in a sealed resin inlet tank.
A second type of system performs resin infusion by starting with solid or putty-like resin underneath the envelope. During resin infusion, a pressure differential is applied across the envelope, resulting in compaction pressure on the reinforcement and the resin. This compaction pressure causes resin to flow into the reinforcement.
This second type of system offers several advantages over the system that injects liquid resin. It can infuse resin at a relatively higher flow rate. Moreover, the second type of system eliminates tubing systems that convey the liquid resin from the inlet tank, thereby reducing complexity of the resin infusion and also eliminating a source of vacuum leaks. The second type of system also enables the use of relatively higher viscosity resins.
The second type of system suffers from not being able to control the pressure and rate of resin infusion independently of compaction pressure on the reinforcement. It would be desirable to overcome this drawback.